Before you ask for help, you need to do several things:
If you see errors in your log like NoSuchMethodError, NoClassDefFoundError, VerifyError, or basically any java Throwable ending with “Error”, followed by a package name that includes the word “api”, you probably have an api error. You either need to update all your mods to very latest version, wait for mods to update, or attempt to remove the api folder from any third party mod zips. It should be noted that removing the API folder only has a very slim chance of fixing the issue.
After all self diagnosing has been exhausted, we ask that you utilize the issue tracker on GitHub to check if there's an existing issue that's already been reported. If you do not see it listed, feel free to post a new issue there.